1. Field of the Invention
The invention relates generally to a screw nut for the rigid fixation of a bone screw to a bone plate having plate holes, whereby the screw nut has a generally cylindrical body with a central axis, a central hole with a hole axis and an internal thread designed to receive said bone screw, a lower section, an upper section, a lower side and an upper side.
2. Description of the Prior Art
Stable fixation is a prerequisite for successful treatment of complex deformity, nonunion, and fracture. Unfortunately, not all situations allow for the application of basic principles, namely creating a construct inherently stable enough so that the bone protects the implant. Osteoporotic bone or bone with abnormally thinned cortices due to failed fixation or previous infection may not allow lasting screw purchase, thus stability gained by applying compression across the fracture site is quickly lost as the thin bone resorbs around the screws. This loss of compression is rapid, since loosening of the screws allows early toggling at the plate/screw interface.
Most types of bone plates which are commonly used in osteosynthesis are fixed solely by means of bone screws to the bone. Since the bone screws are only secured to bone, there is no rigid fixation thereof to the bone plate. Therefore a loosening of the bone screws in the bone or a resorption of the bone can easily lead to a loosening of the bone plate.
It is already known in plate osteosynthesis that a screw nut may be used on the opposite cortex to fix the screw and therefore, the plate to the bone. This method does not produce a direct fixation between bone screws and plate, but only compression of the bone located between the screw nut and the plate and perforated by the bone screw.
In a variety of situations it is desirable to achieve a rigid fixation between bone screws and bone plate in order to avoid subsequent loosening. It is already known from EP-A 340.223 to anchor by means of frictional adhesion alone a bone screw with a specially designed conical head in a bone plate having corresponding conical holes. In this prior art device both holes and screw heads must conform to a specific taper of the conical configuration in order to obtain a rigid fixation between them.
It is also known from the EP-A 340.223 that the projections on the underside of the bone plate reduces the surface area coming into contact with the bone and produces a theoretical improvement in vascularity to the bone. Projections and bone plate are forming, however, an inseparable unit, which can only be used as such and which does not allow for any individual adaptation.
Therefore, the known prior art devices do not offer the versatility sometimes required for optimal treatment in plate osteosynthesis.
The present invention addresses the problems associated with the prior art devices and provides for maximum versatility and adaptability and accomplishes this with a single component device which can be used with any type of bone plate and bone screw and at any selected position (hole) of the plate and which fixes the screw to the plate and acts as a distance holder for reducing the contacting area between bone plate and bone.